[8.x] Add context to subsequent logs.

[chasenyc]

Description:
This adds the ability to add context to all future logs along with the ability to clear that context if desired.

Use case:
A good example of this of how this might be used in middleware, lets say you wanted to bind a uuid to every request:

/**
 * Handle an incoming request.
 *
 * @param  \Illuminate\Http\Request  $request
 * @param  \Closure  $next
 * @return mixed
 */
public function handle(Request $request, Closure $next)
{
    $requestId = Str::uuid();

    Log::withContext(['request-id' => $requestId]);

    $response = $next($request);

    $response->header('request-id', $requestId);

    return $response;
}

Then when parsing logs one would be able to trace all logs pertaining to a specific request with ease. Another example where this would be very useful would be something like adding an authenticated user to the context that would be tied to all following logs.

[shivragjbi]

LGTM!

[GrahamCampbell]

This will not work right with octane, surely, and also with queue jobs. Stuff will bleed across requests.

[taylorotwell]

@GrahamCampbell we will patch that up.